1. Field of the Invention
The present invention relates to an image forming apparatus in which an electrophotographic process or an electrostatic recording processing is used and a developing device for use in the image forming apparatus, particularly to image forming apparatuses such as a copying machine, printer, and facsimile machine, and a developing device for use in the image forming apparatus.
2. Related Background Art
In a conventional image forming portion in image forming apparatus such as a copying machine, an image forming process comprising: charging a photosensitive member as an image bearing member by a charging device; exposing an image of an original to light in an exposure position by an exposure optical system to form an electrostatic latent image on a peripheral surface of the photosensitive member; developing the electrostatic latent image formed on the peripheral surface of the photosensitive member by a developing device to form a developer (toner) image; applying a voltage to a transferring device and transferring the toner image to a transferring material; cleaning the photosensitive member with a cleaner after the image is transferred from the photosensitive member; and performing pre-exposure charging to eliminate a remaining charge, is repeated to form the images.
In the aforementioned developing device, a single developer carrying member (hereinafter referred to as a developing sleeve) is disposed at a constant gap from the photosensitive member. In order to regulate the gap, an abutment roller method is generally used in which the gap is determined by a difference between an outer diameter of a rotary regulating member (abutment roller) coaxially disposed with respect to the developing sleeve and an outer diameter of the developing sleeve, and the gap is further ensured by pressing the developing sleeve toward the photosensitive member.
However, the developing device having the single developing sleeve in the conventional image forming portion cannot catch up with a high speed (copy speed-up). In general, for a peripheral speed of the developing sleeve, the developing sleeve rotates at the speed of about 150% of the peripheral speed of the photosensitive member to develop the image. In order to increase the speed, the peripheral speed of the developing sleeve must be set to 200% or more of the peripheral speed of the photosensitive member; otherwise a supply of developer becomes insufficient and a copy density is lowered. However, when the peripheral speed of the developing sleeve is increased, the increased peripheral speed causes fusion bond of the developer by temperature rise of an end of the developing sleeve, and other problems.
Therefore, there has heretofore been proposed a developing device which is provided with a plurality of developing sleeves and used without largely increasing the peripheral speed of the developing sleeve and which can achieve a high speed. In the developing device, positioning means for securing a constant gap between the photosensitive member and each developing sleeve is preferably disposed with high precision in order to maintain developing properties. However, in the conventional developing device, it is difficult to dispose the developing sleeves to be close to one another.